Solar cells, which receive light and convert the light into electric energy, are classified into the bulk type and the thin film type according to the thickness of the semiconductor. The thin-film solar cells have a semiconductor layer having a thickness equal to or less than several to several tens of micrometers and are classified into the Si thin film type and the compound thin film type. The compound thin film solar cells are further classified into several types including the II-VI family compound type and the chalcopyrite type, and some compound thin film solar cells have been commercially available. In particular, the chalcopyrite solar cell is referred to also as CIGS (Cu(InGa)Se) thin film solar cell, CIGS solar cell or I-III-VI family solar cell after the material used therefor.
The chalcopyrite solar cell has a light absorbing layer made of a chalcopyrite compound and is characterized by its high efficiency, insusceptibility to deterioration by light (deterioration with age), high radiation resistance, wide light absorption wavelength range, high absorption coefficient and the like. Research and development for the mass production of the chalcopyrite solar cell are now being conducted.
FIG. 1 shows a cross-sectional structure of a typical chalcopyrite solar cell. As shown in FIG. 1, the chalcopyrite solar cell comprises a glass substrate, a lower electrode thin film formed on the glass substrate, a light absorbing layer thin film containing copper, indium, gallium and selenium, a buffer layer thin film formed on the light absorbing layer thin film, and an upper electrode thin film. When the chalcopyrite solar cell is irradiated with light, such as sunlight, electron-hole pairs are generated, and at the interface between the p-type semiconductor and the n-type semiconductor, the electrons (−) move to the n-type semiconductor, and the holes (+) move to the p-type semiconductor. As a result, an electromotive force is generated between the n-type semiconductor and the p-type semiconductor. If leads are connected to the electrodes in this state, a current can be drawn to the outside.
FIGS. 2 and 3 are diagrams for illustrating a process of manufacturing a chalcopyrite solar cell. First, a Mo (molybdenum) electrode serving as a lower electrode is formed by sputtering on a glass substrate, such as soda lime glass. Then, as shown in FIG. 3(a), the Mo electrode is split by laser irradiation or the like (first scribing). After the first scribing, shavings are washed away with water or the like, and copper (Cu), indium (In) and gallium (Ga) are deposited on the Mo electrode by sputtering to form a precursor. The precursor is placed in a furnace and annealed in an H2Se gas atmosphere to form a chalcopyrite light absorbing layer thin film. The annealing step is commonly referred to as gas phase selenidation process or simply as selenidation process.
Then, an n-type buffer layer of CdS, ZnO, InS or the like is formed on the light absorbing layer. The buffer layer is typically formed by sputtering or chemical bath deposition (CBD), for example. Then, as shown in FIG. 3(b), the buffer layer and the precursor are split by laser irradiation or with a metal needle, for example (second scribing).
Then, as shown in FIG. 3(c), a transparent electrode (TCO) of ZnOAl or the like serving as an upper electrode is formed by sputtering. Then, as shown in FIG. 3(d), the TCO, the buffer layer and the precursor are split by laser irradiation or with a metal needle, for example (third scribing). In this way, a CIGS thin film solar cell is completed.
The solar cell manufactured as described above is called “cell”. In practice, a plurality of cells are grouped into a package and processed as a module (panel). Each scribing step separates the solar cells into a plurality of stages of solar cells arranged in series. Changing the number of the stages can change the voltage of the cells.
Such a conventional chalcopyrite solar cell has a glass substrate. This is because the glass substrate is an insulator, easily available and relatively inexpensive, and has high adhesion to the Mo electrode layer (lower electrode thin film) and a smooth surface. In addition, sodium in glass is advantageously diffused into the light absorbing layer (p layer) to raise the energy conversion efficiency. However, glass has a low melting point, and high annealing temperature cannot be used in the selenidation step, so that the energy conversion efficiency is low. Furthermore, the glass substrate is thick and heavy, so that a large-scale manufacturing facility is needed, and the completed product is inconvenient to handle. Furthermore, the glass substrate can hardly be deformed, a mass production process, such as roll-to-roll process, cannot be used.
In order to overcome the disadvantages, there has been proposed a chalcopyrite solar cell that uses a polymer film substrate (see the patent literature 1, for example). Furthermore, there has been proposed a technique of forming a chalcopyrite solar cell structure on a stainless steel substrate having a silicon oxide or ferric fluoride layer on upper and lower surfaces thereof (see the patent literature 2, for example). Furthermore, there has been proposed a technique using glass, alumina, mica, polyimide, molybdenum, tungsten, nickel, graphite or stainless steel as a chalcopyrite-based substrate material (see the patent literature 3, for example).
Patent literature 1: Japanese Patent Laid-Open No. 5-259494
Patent literature 2: Japanese Patent Laid-Open No. 2001-339081
Patent literature 3: Japanese Patent Laid-Open No. 2000-58893